The present invention relates to a system and a method for monitoring efficiency of an exhaust gas aftertreatment device, and more particularly, to estimating a change in the oxidant storage capacity of a Lean NOx Trap due to sulfation.
Internal combustion engines are typically coupled to an emission control device known as a three-way catalytic converter (TWC) designed to reduce combustion by-products such as carbon monoxide (Co), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. For lean engine operation, an additional three-way catalyst commonly referred to as a Lean NOx Trap (LNT), is usually coupled downstream of an upstream three-way catalyst. The LNT stores exhaust components such as, for example, oxidants, when the engine is operating at a lean air-fuel ratio, and releases and reduces (purges) them when the engine is operating at a rich or stoichiometric air-fuel ratio.
Over time, the ability of the LNT to store exhaust components can decrease due to factors such as sulfur deposits (SOx) from the fuel. Therefore, when the LNT storage capacity is sufficiently reduced, a SOx purge has to be performed. Typically, the catalyst is heated and engine air-fuel ratio is changed to rich for SOx release and reduction. Since SOx purges result in fuel economy penalties, it is desirable not to purge unnecessarily. Thus, in order to maintain adherence to emission standards and obtain fuel economy benefits of a lean burning engine, it is desirable to monitor the capacity of the LNT to store oxidants, such as oxygen or NOx, for example.
The inventors herein have recognized a need for a more precise and robust system and method of estimating the LNT oxidant storage capacity. In particular, oxidant storage capacity of the LNT varies with changes in the LNT operating temperature achieved during normal driving conditions. Therefore, obtaining an estimate at just one operating temperature point will not provide an accurate result.
In solving the above problem, a system and a method are provided for monitoring efficiency of an exhaust gas aftertreatment device by correlating it to the device""s capacity to store oxidants over an operating range.
In carrying out the above solution, features and advantages of the present invention, a system and a method for monitoring an efficiency of an exhaust gas aftertreatment device disposed in an exhaust passage of an internal combustion engine, include: generating a first device efficiency estimate by monitoring its oxidant storage capacity over on operating range of the device; subsequently generating a second device efficiency estimate by monitoring its oxidant storage capacity over said operating range; and comparing said first estimate to said second estimate.
The present invention provides a number of advantages. For example, monitoring oxidant storage capacity as a function of the LNT operating temperature allows for much more accurate estimate of the LNT efficiency than the one obtained at a single temperature point. Further, results obtained by using more than one data point are much more robust against various noise factors. This prevents unnecessary purges allowing for more lean running time, and increase fuel economy. Also, improved emission control can be achieved due to improved accuracy.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.